In many discrete or integrated circuits, precise and, above all, temperature-independent or almost temperature-independent resistors are a substantial element to guarantee the needed accuracy of the circuit over the entire permitted temperature range.
The electrical resistance of most materials has a relatively high temperature dependence. In metals, the resistance generally increases with increasing temperature, with semiconductors, it decreases. The temperature coefficient of resistance, which in English is also called temperature coefficient of resistance (TCR) and which is on the order of 4,000 ppm/° C. (ppm=part per million) for typical metals such as aluminum, is a measure for the temperature dependence. Only some special materials have a very small temperature dependence of resistance, with those, the TCR is close to 0 ppm/° C., depending on the manufacturing method. Examples for such materials are various alloys such as NiCr, NiCrAl, SiCr or compounds such as TaN.
In CMOS processes (CMOS=complementary metal oxide semiconductor), the wiring of the devices is made, as a rule, through several metal planes connected among each other by contacts. FIG. 4 shows a schematic design of the wiring planes of a standard CMOS device, or process, by example of a three-sheet metallization. The CMOS device comprises four sheets, a first sheet 410 with polysilicon conductive traces 412, a second sheet 420 with the first metal plane, or the first plane of metal conductive traces 422, a third sheet 430 with the second metal plane 432, or the second plane of metal conductive traces, and a fourth sheet 440 with a third metal plane 442, or a third plane of metal conductive traces, the conductors, i.e. the polysilicon conductive traces 412 as well as the conductive traces of the first metal plane 422, the second metal plane 432 and the third metal plane 442 being isolated by an isolator 450 of silicon oxide, and wherein the polysilicon conductive traces 412 are further connected to the conductive traces of the first metal plane 422 by a contact 414 and the conductive traces of the first metal plane 422, the second metal plane 432 and the third metal plane 442 are connected to the respective next metal plane by contacts 424 and 434.
For many applications, particularly in the field of integrated circuits fabricated in the so-called CMOS technology, temperature-independent resistors are needed as reference elements. However, all the resistor layers used as a standard in the CMOS process have TCR values significantly too high, so that they cannot be used for highly precise applications. Thus, in some cases, specially extended CMOS processes are employed, in which thin-film resistors are integrated with the above-mentioned materials. In association with CMOS processes, however, these materials have serious disadvantages. On the one hand, apparatuses for depositing and patterning these materials are not present in the common fabrication lines, only TaN is here an exception in processes with very small structure sizes. Therefore, their employment is associated with significant additional costs. On the other hand, most of the materials have a potentially negative influence on the CMOS devices. If contaminations which can be caused by these special materials come into the range of the devices, they can lead to significant parameter changes or even to a failure of the circuits.